1. Field of the Invention
This invention relates broadly to the field of tubular connectors and more particularly to an extended life marine coupling for marine risers or tension leg platform tendons.
2. Description of Prior Art
As the search for hydrocarbons is extended into deeper waters, alternatives to the conventional marine platform structure for drilling and production operations have been developed. One such deep water alternative is the tension leg platform (TLP). A tension leg platform is a buoyant marine structure or work platform that is moored by a plurality of substantially vertical tendons or tethers that link a submerged anchor base and the floating platform and which are maintained in constant tension. Each tether is preferably formed by a plurality of tension legs which are preferably elongated tubular members. The tethers are disposed in an array or pattern which limits undesired platform movement to maintain the drilling and production equipment on the platform properly positioned on the water surface relative to the submerged wells.
The design requirements for tubular tension leg connectors are related to those for marine drilling and production risers, but are more critical, particularly in fatigue as the tension legs may remain in service for a decade or more. Each tubular tension leg can be formed of a plurality of mechanically end-to-end connected tubular lengths for holding the buoyed platform against undesired movement. Due to the constantly changing force loading of the tension legs resulting from wind, waves and water current, alternating stresses are encountered which induce fatigue damage of the mechanical connectors for the tubular lengths.
Due to the magnitude of the tensile load carried, the tubular tension legs normally have an outer diameter of 10-30 inches and with a relatively thick radial wall thickness. The sheer bulk of the legs provides problems in handling, manufacturing and installation of the tension legs which are formed in long lengths and coupled together. The couplings or connectors for joining the tubular tension leg lengths are subjected to the same mean and alternating force loadings as are encountered by the tension leg lengths.
Fatigue failure is affected in load transmitting tubular member connectors by two basic factors: (1) alternating tensile stresses, and (2) stress concentration points.
The prior art has shown that tensile stress, not compressive stress is usually critical to the fatigue lifetime of a tubular connection. Crack initiation and propagation, which lead to fatigue failure, occur sooner in members subjected to higher mean tensile stresses and to higher amplitude alternating tensile stresses.
The prior art has also shown that fatigue failures invariably originate at higher localized points of stress concentration.
3. Identification of Objects of the Invention
An object of the invention is to provide a tubular member mechanical coupling having an extended fatigue life.
Another object of the invention is to provide a load transmitting tubular member connector which includes members of different stiffness whereby a change in tensile load to the coupling results in a relatively low change in tension in one of the members, the other members being essentially in compression, whereby the lifetime of the coupling is increased due to the relatively small change in tension of said one member.
It is still another object of the invention to provide a load transmitting tubular member connector which includes a circumferentially continuous load carrying member having a thick section in which a portion of the alternating tension load applied to the tubular members is essentially harmlessly dissipated by cyclic compressive strain.
It is another object of the invention to provide a tubular member connector of simple design which avoids abrupt changes in geometry.
Another object of the present invention is to provide a tubular member connector of simple design with resultant economies of manufacturing.
Yet another object of the invention is to provide a tubular member connector of relatively small mass with resultant economies of operating efficiency, smaller size, easier rig handling, and cost.